This book is the result of a 4-year research project conducted at the Faculty of Law of the University of Luxembourg. It explores the legal value and enforceability of tax circulars and tax rulings in Luxembourg domestic law in light of the principle of legitimate expectations and related principles. After studying the historical roots of both interpretative acts, this research questions the level of protection taxpayers enjoy when relying on circulars and tax rulings and contains a review of decades of administrative case-law to assess the judicial discourse on taxpayers’ rights to certainty.This book further investigates the case of circulars and tax rulings that contain interpretations of tax laws that are contrary to the law (contra legem) and builds upon the existing normative framework to introduce proposals addressing issues of uncertainty and inequality taxpayers are likely to suffer when relying on such interpretative acts.Prix Pierre Pescatore de la Faculté de Droit de Luxembourg (École doctorale de droit).